1. Field of the Invention
The present invention relates generally to an optical filter used for modifying spectral power distribution of a lamp, particularly to an optical filter capable of modifying the spectral power distribution of a high color temperature halogen lamp from 3500 K to 6500 K to simulate CIE standard daylight illuminant with very high color rendering index, ultraviolet light free if required, and much less heat emission.
The optical elements for modifying the spectral power distribution of a light source are colored glass filters, thin film filters, and liquid filters. The most used element is a single colored glass filter to modify the spectral power distribution of a lamp to simulate the Commission Internationale de L'eclairage (hereinafter “CIE”) standard daylight illuminant. The colored glass plates can be arranged in serial and/or parallel arrangements. However, practically the colored glass plates in serial and parallel arrangements are rarely used for modifying the spectral power distributions due to the complexity of design, difficulty of fabrication, and high cost. At present, thin film filters and liquid filters are rarely used for simulating CIE standard daylight illuminant.
Currently, commercially available full spectral CIE daylight simulators are made by one color temperature conversion glass filter with a halogen lamp. The colorimetric indexes of such CIE daylight simulators are generally not high, and the spectra of the CIE daylight simulators do not match that of CIE D illuminant well.
2. Description of the Prior Art
As shown in Hennessey U.S. Pat. No. 3,504,959, a light filter can be directly coated on the front glass of a lamp to modify the spectral power distribution of the lamp.
As shown in Ota U.S. Pat. No. 6,693,373, a colored filter can be easily mounted in a xenon lamp or a halogen lamp, and is capable of making the lamps emit stable bluish light for automobile lighting. However, the capability for modifying the spectral power distribution of both filters is not sufficient for simulating CIE standard daylight illuminant.
As shown in the Kageyama U.S. Pat. No. 4,865,405 and the Kyogoko U.S. Pat. No. 4,659,178, some optical filters have been disclosed for color correction. Such optical filters comprise one or two plates of colored glass and one or two interference multiplayer film. The interference multiplayer films are coated on the colored glass plates. The plates have the narrow band-pass or heat absorbing characteristics. The glass plates and the interference multiplayer films are in series. The optical filters are capable of providing narrow pass bands for color correction of color devices. However, the optical filters are not capable of modifying the spectral power distribution of a lamp to simulate CIE standard daylight illuminant.
As shown in Eligehausen U.S. Pat. No. 4,072,856, a daylight-simulating incadescent lamp light fixture was disclosed for medical and dental use. The fixture has a reflector that selectively reflects the light from a 3000 K incadescent lamp to approximately match daylight.
At present, several high color temperature halogen lamps are commercially available from 3500 K to 6500 K as daylight lamps as shown in the McGuire U.S. Pat. No. 6,611,082. The high color temperature of the halogen lamp is mainly achieved by the specially designed reflector that predominately reflects short wavelength visible light. At high color temperature over 4200 K, the spectral power distribution of a halogen lamp significantly differs from that of CIE daylight illuminant. Such type of high color temperature halogen lamp cannot be used as a standard CIE daylight simulator for colorimetric purposes, such as visual color match and color grading. Nevertheless, some of the high color temperature halogen lamps are new types of light sources for simulating CIE standard daylight illuminant with high colorimetric indexes.
As discussed in the article by Liu et al. entitled “Optimization Algorithm for Designing colored Glass Filters to Simulate CIE Illuminant D65”; Color Research and Application, Vol. 16, No. 2, pp. 89-96 (1991), Liu et al. used an optimization algorithm for designing colored glass filters in parallel arrangement to simulate CIE illuminant D65. The D65 daylight simulators have high color rendering index. The spectral power distributions of the D65 simulators match that of the CIE standard illuminant D65 very well. However, the filters in parallel arrangement are difficult to design, complex to fabricate and are made at a high cost. The D65 daylight simulators with colored glass filters in parallel arrangement have a high academic value, but little practical and industrial application.
As shown in Keller U.S. Pat. No. 5,149,182, optical filters with glass plates in series have been used for modifying the spectral response of detector for matching the spectral response of the human eye and other CIE color matching functions for the purposes of colorimetric measurement and photometric measurement. An optical filter with color glass plates in series has been pursued for simulating CIE standard daylight illuminant with high colorimetric indexes and better spectral match. It is therefore an object of the invention to provide an optical filter for simulating the CIE standard daylight illuminant at different color temperatures.